(1). Field of the Invention
The present invention relates to a curtain coating apparatus for use in coating photographic printing paper, magnetic recording paper, heat-sensitive recording paper, pressure-sensitive recording paper, ink jet image-receiving paper, heat transfer image-receiving paper and pigment-coated paper, specifically to a curtain coating apparatus which inhibits an excess coated part produced in starting and stopping coating, and a coating method.
(2). Description of the Prior Art
A curtain coating apparatus is a coating apparatus of a so-called pre-metered coating type in which metered coating is carried out prior to a coating operation, and unlike a coating process of an after-metered coating system in which metered coating to a prescribed amount is carried out after feeding an excess coating liquid to a web, the liquid composition of the coating liquid does not change with the passage of time, and a coated matter having a stable quality can be obtained over an extended period of time. Further, since multi-coating is possible and the upper limit of the coating speed is high depending on the coating conditions, coated matters can efficiently be obtained.
Thus, because of many advantages of the curtain coating apparatus, it is used in various coating fields such as photographic printing paper, magnetic recording paper, heat-sensitive recording paper, pressure-sensitive paper, ink jet image-receiving paper, heat transfer image-receiving paper and pigment-coated paper.
In general, coating in curtain coating is started by disposing a shielding member between a curtain film which has been formed in advance and a web to capture the whole coating liquid with the shielding member and removing the shielding member to strike the curtain film against the web. The coating is stopped by bringing the shielding member back to a position where it used to be present before starting the coating to shield again the curtain film from the web. As shall be described below in detail, in such operations as starting and stopping the coating, a part (hereinafter referred to as an excess coated part) where an excess coating liquid is fed to the web is formed, wherein the excess coated part passing through a drying zone while the drying is incomplete stains a roll with the coating liquid and makes the coating operation difficult as well as bringing about a serious defect in terms of quality.
The excess coated part in starting the coating is formed in a different step from immediately after stopping the coating. That is, the coating is started by moving and removing the shielding member disposed between the web and the curtain head, wherein the curtain film is cut with the tip of the shielding member, and the end part of the curtain film forms a liquid film thicker than the curtain film by virtue of the surface tension of the coating liquid. When this thick end part strikes against the web, the excess coated part where the coated amount becomes excessive at a coating-starting part is formed.
The excess coated part formed on the web stays in an undried condition in ordinary drying and transfers the excess coating liquid at a part where the web contacts a roll used for transporting the web. Further, a part of the coating liquid transferred on the roll is transferred again to a coated layer on a web surface, or in contrast with this, it disturbs and peels the ordinary coated layer to bring about coating fault. Accordingly, the formation of the excess coated part exerts an adverse effect on the quality of the coated matter. Further, in order to remove the coating liquid which has contaminated the roll, the apparatus has to be stopped, and therefore the production efficiency is obliged to be notably reduced.
On the other hand, the shielding member is inserted into the curtain film in stopping the coating to cut again the curtain film with the tip of the shielding member. After cutting the curtain film with the tip of the shielding member, the curtain film contracts due to the surface tension thereof to become stick-shaped liquid sagging larger than a liquid film thickness, which drops on the web to form the excess coated part. Further, when cutting the curtain film, the coating liquid adheres to the shielding member to become a droplet, which falls on the web to form the excess coated part. Accordingly, the roll is stained with the coating liquid at the excess coated part as is the case with starting the coating. Also in this case, the cleaning work of the roll is inevitably required and results in bringing about a reduction in the production efficiency.
Means for inhibiting the formation of the excess coated part includes a method in which the shield member used in the operations of starting and stopping the coating is brought as close to the web as possible to minimize the excess amount at the excess coated part. While this method is effective, a function of a receiver for receiving some amount of the coating liquid is requested to the shielding member, and therefore a fence having a fixed height is required to be provided at the edge of the shielding member. Accordingly, even if the shielding member is brought close to the web, a point (hereinafter called a curtain film-cutting point) where the curtain film is cut with the tip of the shielding member in order to start or stop the coating becomes high to some extent. On the other hand, in a method of starting and stopping coating used in U.S. Pat. No. 3,508,947, as shown in FIG. 5 of the attached drawings, a preformed curtain film 2, flowing toward a web 3 supported by a roll 4, is intercepted by a deflector 15. The coating liquid forming the curtain film 2 is directed into a receiver 16 by the deflector 15 to stop the coating operation. The coating operation is commenced by rotating the deflector 15 about a pivot axis 18 and keeping the deflector 15 spaced from the curtain film 2, thus allowing the curtain film 2 to flow into contact with the web 3. To prevent air flow from being developed by running movement of the web 3, a wind interceptor (not shown) is set up slightly in front of the coating point.
In this method, starting and stopping of the coating operation are effected by the deflector 15 crossing the curtain film diagonally, but the deflector itself is not required to function as a receiver. However, in order to cause the liquid to fall into the receiver 16 after flowing along the deflector 15, the deflector 15 has to be inclined and a cutting point of the curtain film must be kept a distance from the web 3. As a result, the method requires a distance of several centimeters between a fall-starting point of the film and the web, and therefore does not provide for inhibiting the formation of the excess coated part.
In the methods of starting and stopping curtain film coating described in Japanese Patent Application Laid-Open (through PCT) No. Hei 2-503884 and U.S. Pat. No. 4,851,268, a trapping pan 17 as shown in FIG. 6 is used, and, when coating is commenced, the point of cutting the curtain film 2 may be only a few millimeters above the web 3. However, very slight shaking caused by moving the trapping pan 17 provides a large possibility that the coating liquid received in the trapping pan 17 would be spilled from the trapping pan 17. Actually, it is difficult to cause the function of the receiver to be sufficiently well compatible with having a distance of some millimeters between the cutting-point of the curtain film 2 and the web 3, and as a result, the excess coated part ends up being formed to such an extent that problems remain.
In the method described in Japanese Patent Application Laid-Open (through PCT) No. Hei 2-503884, it would be possible to stop coating without forming an excess coated part by removing the received coating liquid from the trapping pan 17 in advance of stopping the coating operation so as to provide some measure against spilling of the coating liquid from the trapping pan 17.
However, when starting coating, the coating liquid is liable to spill out while moving the trapping pan 17 receiving the coating liquid between the curtain film 2 and the web 3. Therefore, the excess coated part would be formed as well in the method described in Japanese Patent Application Laid-Open (through PCT) No. Hei 2-503884.
That is, in the curtain coating in which a coated layer is formed by causing the curtain film to flow down to strike against the web, it is very difficult to start the coating without forming the excess coated part, and methods therefor have not been present.